The present invention relates to a color picture tube having a mask-frame combination which includes a mask and a frame formed of first and second materials, respectively, and connected by a plurality of welding spots.
In color picture tubes, three primary color pictures are pixelwise superimposed by juxtaposing the color points pertaining to one pixel. At a sufficiently great viewing distance, the pointed structure will no longer be perceived by the eye and the colors will be mixed in an additive manner. The electron beams of the three beam generating systems of color picture tubes or color screens impinge on the phosphor stripes or phosphor dots that are arranged on the phosphor screen. To make sure that only the electron beam of one of the three electron-beam generating systems for the three primary colors impinges on the phosphor stripes or phosphor dots of the corresponding color, use is made of shadow masks which "shadow" the two other electron beams. About 80% of the electrons that are emitted by the beam generating systems impinge on the shadow mask. This will heat and thus expand the shadow mask, whereby holes might be displaced with respect to their position relative to the phosphor stripes or phosphor dots.
The shadow mask or also mask consists of a thin, low-carbon iron sheet or iron alloys. Nowadays, the masks are often made from iron-nickel alloys which in comparison with iron have a very small thermal expansion at low temperatures due to magnetostriction. The mask has etched thereinto about 400,000 holes which are assigned to the approximately 600 color stripe triples or to the approximately 400,000 color point triples, i.e., phosphor stripes or phosphor dots arranged in groups of three with a respective red, green and blue luminescent stripe or dot.
To give the thin shadow mask a certain mechanical stability and in order to make it manageable, the curved shadow mask is welded into or onto the mask frame. In modern color picture tubes, the frame is also made from thin sheet metal. The shadow mask with frame and suspension must be at least so stable that the picture tube will withstand a maximally tolerable acceleration of 35 g. Since iron-nickel alloys are much more expensive than iron, the mask frame (or also frame) is made from iron, whereas the mask consists of an iron-nickel alloy.
When such masks are drawn from sheets, a mask flange will be formed and the mask surface will become arched. The mask flange is connected by a plurality of welding spots to the frame. It is difficult to connect a mask and a frame that are made from materials having different coefficients of thermal expansion. During heating of the mask-frame combination the mask might become deformed, and such deformation will change the positions of the holes in the mask relative to the spatial positions of the associated phosphor stripes or phosphor dots. Particularly on account of the different thermal expansion characteristics of the mask and the frame during thermal manufacturing processes and during operation of a color picture tube or a color screen, it is only a loose coupling that is desired. On the other hand, a great stiffness is desired for the connection established between the mask and the frame, so that the mask will not become deformed upon impacts and will not be induced to vibrate by the sound waves of the loudspeaker of a television set.
Attempts have been made to simultaneously meet such conflicting demands by using stiffening beads in the mask, decoupling slots or recesses in the mask frame and by the placing of the welding spots.
EP-A1 063 322 suggests a solution which is concerned with an iron mask having an iron frame, with the mask being welded outside the frame. As a result, a formerly necessary manufacturing step (temperature treatment (stabilization) in a through-type furnace) can be dispensed with. The positions of the welding spots at the longer and shorter sides of the frame only permit specific areas at each side in addition to the corner points for the application of welding spots for receiving the stresses in the material that arise or remain during temperature treatments (for instance black stabilization, frits) by way of deformations which from a functional point of view are still admissible and remain within the elastic range. Such welding spots must be within a specific height range of the mask edge which, just like the other indicated ranges, depends on the diagonal length of the mask. Moreover, decoupling requires notches, recesses or beads which remove mechanical stresses at both sides from the welding spots provided at the comers of the mask. A surrounding bead which is provided above the mask edge serves the same purpose.
EP A2 0518 249 suggests another solution for masks of iron-nickel alloys with iron frames, wherein the mask is welded outside the frame. A total of twelve welding spots are used for connecting the frame and the mask so as to weld the mask flange to the outside of the frame. The welding spots are respectively positioned at the corners and approximately in the center of each side. In addition, welding spots are respectively provided at the longer sides between the central welding spot and the welding spot in the corner in an area near the central welding spot. Moreover, the welding spots are placed as close as possible to the free end of the mask flange, and each of the recesses occupies almost the whole area between the welding spots.